Applications of Photovoltaic Technologies

Referenced website:

http://www.udel.edu/igert/pvcdrom/

http://solarpv.itri.org.tw/memb/main.aspx

2

Why Solar Cells?

• Finite fossil fuel supply

• Less environmental damage

• No radiation risk (meltdown)

• Nearly infinite supply of FREE energy

• Sun gives us 32 x1024 joules a year,

• Cover 0.1% of the Earth’s surface with 10% efficient solar cells with an efficiency of would satisfy our present needs.

3

Greenhouse Effect

• Human activities have now reached a scale where they are impacting on the planet's environment and its attractiveness to humans.

4

Spectrum of light

c

hhE

h: Planck’s constant 6.626×10-34 (J-s)

ν: frequency (s-1)

λ: wavelength (m)

c : light speed 3.0× 108 (m/s)

5

Atmospheric Effects

Hu, C. and White, R.M., "Solar Cells: From Basic to Advanced Systems", McGraw-Hill, New York, 1983.

6

Solar Radiation

Power emitted from Sun =3.8×1023 (kw)Power direct to Earth=1.8×1014 (kW)Solar constant=1353 W/m2

T=5762 K

7

Air Mass (AM)

• AM0 : The standard spectrum outside the Earth's atmosphere.

• AM 1: Light incident with the angle of 0 degree.

• AM 1.5: Light incident with the angle of 48 degree.

cos

1AM

687.0

7.01353AM

DI

DG II 1.1•ID : Direct beam intensity (W/m2)

•IG : Global irradiance (W/m2)

Meinel A.B. and Meinel M.P., "Applied Solar Energy", Addison Wesley Publishing Co., 1976

Intensity

8

Standard Solar Spectra

9

• The AM1.5G

Global spectrum is designed for flat plate modules

and has an integrated power of 1000 W/m2 (100

mW/cm2).

• The AM1.5 D

The direct plus circumsolar spectrum has an

integrated power density of 900 W/m2.

Standard Solar Spectra-cont.

10

Part of periodic table

II III IV V VI

B C(6)

Al Si(14) P S

Zn Ga Ge(32) As Se

Cd In Sb Te

11

Compound semiconductors

• Elemental semiconductors: Si, Ge

• Compound semiconductors: GaAs, InP

• Ternary semiconductors: AlGaAs, HgCdTe

• Quaternary semiconductors: InGaAsP, InGaAlP

Elemental IV Compounds

Binary III-V Binary II-VI

Si SiGe AlP CdTe

Ge SiC GaAs CdS

As InP ZnS

GaP CdSe

12

Crystal Structures

Polycrystalline

AmorphousCrystalline

In a crystalline solid atoms making up the crystal are arranged in a periodic fashion

Some solids are composed of small regions of single crystal material, known as polycrystalline.

In some solids there is no periodic structure of atoms at all and called amorphous solids

13

Commercial Si solar cells

SINGLECRYSTAL POLYCRYSTAL AMORPHOUS

14

Photoelectric effect

Metal

Photon Electron

Photon is a particle with energy E = hv

•Semiconductor

EgPhoton

Eph( hv)>Eg

15

Direct and indirect semiconductor

High absorption probability Low absorption probability

Ev

E

P

Ec

Direct Semiconductor

photon

Ev

E

P

Ec

Indirect Semiconductor

phonon

photon

GaAs; InP etc. c-Si

16

Metal-insulator-conductor

• Metal →CB and VB overlap,

• Insulator and semiconductor CB and VB are separated by an Eg (energy band Eg).

• Eg for Si is 1.1242eV (semiconductor) ;5eV for diamond (Insulator)

Filled States (VB)

Empty States (CB)Eg

metal semiconductor insulator

17

Absorption of Light

• Eph < EG Photons with energy Eph less than the band

gap energy EG interact only weakly with the

semiconductor, passing through it as if it were

transparent.

• Eph = EG have just enough energy to create an electron

hole pair and are efficiently absorbed.

• Eph > EG Photons with energy much greater than the

band gap are strongly absorbed

18

N- and P-type

• Addition of impurities with three valence electrons results in available empty energy state, a hole

• B, Al, In, Ga (Acceptor impurities)

•Addition of impurities with five valence electrons results an extra electron available current conduction

• P, As, Sb (donor impurities

19

Physics of Photovoltaic Generation

※Ehp > EG

※Electron-hole pair (EHP) .

※Electrons go to negative electrode; hole to positive electrode.

n-type semiconductor

p-type semiconductor

+ + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - -

Physics of Photovoltaic Generation

Depletion Zone

21

Solar Cell-structure

• A solar cell is a P-N junction device

• Light shining on the solar cell produces both a current and a voltage to generate electric power.

Busbar

Fingers

Emitter

Base

Rear contact

Antireflection coating

Antireflection texturing

(grid pattern)

22

Solar cell structure

• How a solar cell should look like ?

It depends on the function it should perform, it should convert light into electricity, with high efficiency

• It should be a P-N junction

•P-type

•N-type

• There should be ohmic contact at both side

• It should absorb all light falling on itIt should reflect less light Most of the light should go in

• It should convert all absorb light into electricity

23

Minimizing optical losses

• The optical path length in the solar cell may be increased by a combination of surface texturing and light trapping.

•Top contact coverage of the cell surface can be minimized

• Anti-reflection coatings can be used on the top surface of the cell.

• Reflection can be reduced by surface texturing

• The solar cell can be made thicker to increase absorption

•There are a number of ways to reduce the optical losses: .

24

Optical properties of surface

•What are optical losses:

Reflection

Shadowing due to metal contact

Partial absorption

• Photons in the spectrum can generate EHP, ideally all the sun light

• falling on the cell should be absorbed

•Short circuit current (ISC) is usually reduced due to optical losses

• Design criteria for small optical losses :

• Mminimize optical loss

25

•Air, n0

•Semiconductor, n2

•ARC, n1

• The thickness of a ARC is chosen such that the reflected wave have destructive interference this results in zero reflected energy

• The thickness of the ARC is chosen so that the wavelength in the dielectric material is one quarter the wavelength of the incoming wave (destructive interference).

1

01 4n

d

110 n

•n2 > n1 > n0

Choice of ARC

26

Reflection from various combination

• Multilayer structure reduces the reflection losses

• Index of refraction is also a function of wavelength, minimum reflection is obtained for one wavelength

• More than one ARC can be used, but expensive

•Source: PV CDROM - UNSW

27

Surface texturing

• Any rough surface decreases the reflection by increasing the chances of the reflected rays bouncing back on the surface

• Surface texturing can be obtained by selective etching a process by which material is removed by chemical reaction

• Selective etching is based on the concept of different material property in different direction in crystals,

• Etching rate are different in <100> dirn than in <111> dirn

28

Surface texturing

• Chemical etching in KOH results in pyramid formation on the Si surface etching is faster in <100> direction than in <111> direction

• Using photolithography, inverted pyramids can be obtained, which are more effective

•<111> surface

29

Light trapping

2211 sinsin nn

• Rear side reflector or rear side texturing is used to increase the optical path length in solar cell Increased optical path is required for thin solar cell (thin solar cell have higher Voc. It saves expensive Si)

• Total internal reflection (TIR) condition are used to increase the optical path length

•Snell’s law

• (1 for Si is 36 degree)

)(sin1

211 n

n•For TIR

30

Lambertian Rear Reflectors

• Increases the path length by 4n2, very good in light trapping, path ;length increases by about 50

•Random reflector from the rear side

•TIR

• Lambertian reflector is one which reflects the lights in a random direction this together with the front texturing increases the optical path length

31

•P-N junction

Current loss due to recombination

• Recombination areas

Surface recombination

Bulk recombination

Depletion region recombination

• Recombination of carriers reduces both short circuit current as well as open circuit voltage

Bulk semiconductor rear surface

Front surface

•Design criteria: The carrier must be generated within a diffusion length of the junction, so that it will be able to diffuse to the junction before recombining

32

ww

hhd

Emitter

finger and busbar spacing, the metal height-to-width, aspect ratio, the minimum metal line width and the resistivity of the metal

•Top contact

One example of top metal contact design

Design criteria: minimize losses (resistive, shadow)

•

33

Resistive Losses

• Resistive effects (series and shunt resistance) in solar cells reduce the efficiency of the solar cell by dissipating power in the resistances.

• Both the magnitude and impact of series and shunt resistance depend on the geometry of the solar cell and solar cell area

• Resistance are given in Ω-cm2

•IL •If

•Rs

•Rsh

•V

•I

•Solar Cell model

• The key impact of parasitic resistance is to reduce fill factor.

sh

ssL R

IRV

nkT

IRVqIII

)(exp0

34

Resistive Losses: Series resistance, Rs

•1. the movement of current through the emitter and base of the solar cell

•3. resistance of the top and rear metal contacts

•2. the contact resistance between the metal contact and the silicon

•Contributing factors to Rs :

•Bus bar

•Fingers

•N-layer

•p-layer•Base•emitte

r

•M-S contact

35

Contact resistance

•N

•Heavy doping under contact to minimize contact resistance

•Metal to semiconductor contact • Contact resistance losses occur at the

interface between the silicon solar cell and the metal contact. To keep top contact losses low, the top N+ layer must be as heavily doped as possible.

• A high doping creates a "dead layer“.

• Ohmic contact,

• High doping, tunneling contact

36

Sheet resistance

•In diffused semiconductor layers, resistivity is a strong function of depth. It is convenient to a parameter called the "sheet resistance" (Rs).

W

LRs

W

L

t

A

LR

• Rs is called sheet resistance with unit of ohms/square

or Ω/□ (actual unit is Ohms)

•The L/W ratio can be thought of as the number of unit squares (of any size)

• Sheet resistance of a solar cell emitter is in the range of 30 to 100 Ω/□

•W

•L

•t

37

Emitter resistance: Power loss

• t

•P

•N •d

•L

•x

•dx

•d/2

• Zero current flow exactly at midpoint of fingers

• Maximum current density at the finger edge 2max

dJLI

• Resistance dR in infinitesimally thin layer of dx tL

dxdR